carlopmart wrote:
> Les Mikesell wrote:
>> carlopmart wrote:
>>>>>>>> Thanks lars. Correctly, firewall could be the problem, but it isn't. Because
>>>>>>> Ubuntu and Windows 2003/2008 doesn't have problems with it ... and resolves
>>>>>>> perfectly ... And I don't have configured this firewall to accept dns queries
>>>>>>> originating from source port 53 ...
>>>>>>>
>>>>>> What does 'dig' show about your access to the root servers without
>>>>>> forwarders and with and without forcing the query-source port? Compare
>>>>>> it to the Ubuntu system. Maybe there's something wrong with the root
>>>>>> hints file - or maybe your border firewall is blocking all udp to this
>>>>>> box but permitting it to the DNS servers that work.
>>>>>>
>>>>> Thanks Les, but I have checked it before post this problem. Ubuntu and CentOS
>>>>> have the same file to do querys to root servers ...
>>>> And the results of 'dig' on each?
>>>>
>>>>> I have find a temporary solution: reduce the MTU on CentOS server (1440) ...I
>>>>> need to investigate why centOS loses some packages and ubuntu doesn't ....
>>>> Are you routing through tunnels?
>>>>
>>>>
>>> No, all hosts (firewall and CentOS DNS server) are connected to GByte network.
>> That's not where the problem is. Since you are working with forwarding
>> on, the problem has to be when you try to go directly to the internet
>> over UDP so it would be at the firewall or border router. When DNS
>> fails, it will retry with TCP and that might be why it eventually works.
>
> That's not possible, because firewall only permits DNS querys over UDP ...
I'd advise following the standards. If the response won't fit in a udp
packet, it has to fail over to tcp.
>
>> Is there anything in the path to the internet that needs a lower MTU
>> (perhaps a DNS line running PPOE)? Or do you have jumbo packets enabled
>> on your Gig NIC?
>
> No, but firewalls have a mtu configured with 1450 on external interfaces ...
Why?
> And if you do need a small MTU, do you have firewalls
>> blocking the ICMP messages that are required to discover that automatically?
>
> Yes, ICMP messages are blocked on firewall, but are blocked for all hosts:
> centos dns servers, ubuntu servers, windows servers ... i don't understand why
> using Ubuntu or windows servers to resolve names works ok and with centos (and
> with either rhel5. I have just check it) doesn't ...
The 'dig' response might give you a hint. But if all other network
operations work OK, I'd still guess it is a firewall setting that you
are missing.
--
Les Mikesell
lesmikesell at gmail.com